You should look into how many people lied about their age and enlisted.
An example: Jack Lucas. Born 1928, enlisted in '42 at 14. Invaded Iwo Jima. Received the Medal of Honor at 17 when he jumped on a grenade and survived. He went back to high school to get his diploma.
A brief Google tells me he enlisted in the British military at 16 in '44 just before D-day. That may be incorrect because there are a few people with his name that served in the Bri’ish military, but the one I found was a tanker.